A long-lost essay by Agatha Christie that was commissioned by the government in 1945 to sing the praises of British crime fiction has finally seen the light of day.

Christie's essay, in which she extols the virtues of the British detective story, has been published for the first time in the UK as the preface to the reissued 1933 collaborative crime novel Ask a Policeman. "I discovered it in 1997 going through her archive but never had an opportunity to publish it," said David Brawn, who publishes Christie at HarperCollins. "Although it was published in a Russian magazine in 1947, it's never been seen in the UK before. She was commissioned to write it by the Ministry of Information in Britain in order to seed it out internationally – it's really a piece of propaganda; they were trying, I guess, to extol the virtues of the British and western way of life, and so the government asked her to write this essay about the crime-writing genre."

The piece sees Christie writing admiringly of Arthur Conan Doyle, "the pioneer of detective writing", before going on to commend John Dickson Carr – "a master magician … the supreme conjurer, the King of the Art of Misdirection" and Ngaio Marsh, "another deservedly popular detective writer".

Christie describes herself modestly, as able to "lay claim at least to being an industrious craftsman" – although she points out that "a more aristocratic title was given to me by an American paper which dubbed me the 'Duchess of Death'" – but she isn't a wholly effective propaganda machine. She retains some incisive criticism for Margery Allingham, who "is inclined to subordinate plot to characters. She is so interested in them that the denouement of the crime sometimes comes rather flatly as inevitable, rather than as a surprising bombshell."

And while Dorothy L Sayers is an "exceptionally good detective story writer and a delightfully witty one", she, too, comes in for some unexpected censure from Christie, who writes that Sayers' creation Lord Peter Wimsey is "an example of a good man spoilt". His face, says Christie, "was originally piquantly described as 'emerging from his top hat like a maggot emerging from a gorgonzola cheese'", but Wimsey sadly "became through the course of years merely a 'handsome hero', and admirers of his early prowess can hardly forgive his attachment to, and lengthy courtship of, a tiresome young woman called Harriet".

"I guess at the time because she didn't expect it to be published in Britain, she doesn't pull her punches," said Brawn. "And then there's a lovely bit at the end where she says she's bored to tears of Poirot."

"My own Hercule Poirot is often somewhat of an embarrassment to me – not in himself, but in the calling of his life. Would anyone go and 'consult' him? One feels not," writes Christie. And later: "Poirot has made quite a place for himself in the world and is regarded perhaps with more affection by outsiders than by his own creator! I would give one piece of advice to young detective writers. Be very careful what central character you create – you may have him with you for a very long time!"

Ask a Policeman, in which the preface now appears, was the second novel written by members of the Detection Club, a group of British authors set up in 1930. The book's title was dreamed up by Milward Kennedy, John Rhode thought out the murder and suspects, and Gladys Mitchell, Helen Simpson, Sayers and Anthony Berkeley lent their detective creations to solve the mystery. The authors then swapped characters – so Berkeley, for example, took Lord Peter Wimsey and Sayers took Berkeley's Roger Sheringham – allowing them to poke playful fun at each other.

"The Detection Club was a dining club for crime writers – they used to raise money to pay for their dinners by writing these novels," says Brawn. Christie, a member of the club, contributed to its first collaborative novel, The Floating Admiral, but sat out of Ask a Policeman, so he decided the novel was the perfect opportunity to bring her long-forgotten essay back into print.

"It does show she clearly did read and was aware of what was going on in crime fiction," said Brawn. "She wasn't writing in isolation." Brawn is in conversation with the Detection Club's current president, author Simon Brett, about the possibility of a modern collaboration between club members, who today range from PD James to Sophie Hannah. "He's really keen on one but it's a complicated thing to do," added Brawn. "But the idea is out there … and it might pay for a few more lunches."